U.S. patent application number 10/903408 was filed with the patent office on 2005-03-03 for tread for a tire.
This patent application is currently assigned to MICHELIN RECHERCHE ET TECHNIQUES S.A. Invention is credited to Vasseur, Didier.
Application Number | 20050049344 10/903408 |
Document ID | / |
Family ID | 27675990 |
Filed Date | 2005-03-03 |
United States Patent
Application |
20050049344 |
Kind Code |
A1 |
Vasseur, Didier |
March 3, 2005 |
Tread for a tire
Abstract
A tire tread comprising a rubber composition based on at least:
(i) a diene elastomer; (ii) more than 50 phr of a reinforcing
inorganic filler; (iii) between 2 and 15 phr of a coupling agent;
(iv) an unsaturated metal carboxylic acid salt. This tread has,
after mechanical running-in of the tire comprising it, a rigidity
gradient which increases radially from the surface towards the
inside of the tread, imparting to the tire an improved compromise
of grip/behavior properties. Use of such a tread for the
manufacturing or recapping of tires. Tires comprising such a
tread.
Inventors: |
Vasseur, Didier;
(Clermont-Ferrand, FR) |
Correspondence
Address: |
Alan A. Csontos
Michelin North America, Inc.
Intellectual Property Department
P.O. Box 2026
Greenville
SC
29602
US
|
Assignee: |
MICHELIN RECHERCHE ET TECHNIQUES
S.A
GRANGES PACCOT
CH
|
Family ID: |
27675990 |
Appl. No.: |
10/903408 |
Filed: |
July 30, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10903408 |
Jul 30, 2004 |
|
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PCT/EP03/01125 |
Feb 5, 2003 |
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Current U.S.
Class: |
524/394 ;
524/430; 524/492 |
Current CPC
Class: |
C08F 279/02 20130101;
C08L 51/04 20130101; B60C 1/0016 20130101; C08L 53/02 20130101;
C08F 287/00 20130101; Y10T 156/1317 20150115; C08L 51/04 20130101;
C08L 2666/08 20130101; C08L 2666/02 20130101; C08L 53/02 20130101;
C08L 21/00 20130101; C08L 2666/02 20130101; C08L 21/00 20130101;
C08L 21/00 20130101; Y10T 152/1018 20150115; B60C 11/00 20130101;
C08K 5/098 20130101; C08K 5/098 20130101 |
Class at
Publication: |
524/394 ;
524/492; 524/430 |
International
Class: |
C08K 003/34; C08K
003/18; C08K 005/04 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 7, 2002 |
FR |
FR 02/01658 |
Claims
What is claimed is:
1. A tire tread comprising a rubber composition based on at least:
(i) a diene elastomer; (ii) more than 50 phr of a reinforcing
inorganic filler; (iii) between 2 and 15 phr of a coupling agent;
and (iv) an unsaturated metal carboxylic acid salt.
2. The tread according to claim 1, wherein the diene elastomer is
selected from the group which consists of polybutadienes, synthetic
polyisoprenes, natural rubber, butadiene copolymers, isoprene
copolymers and mixtures of these elastomers.
3. The tread according to claim 1, wherein the reinforcing
inorganic filler is a siliceous or aluminous filler.
4. The tread according to claim 3, wherein the reinforcing
inorganic filler is silica.
5. The tread according to claim 1, wherein the quantity of
reinforcing inorganic filler is between 60 and 120 phr.
6. The tread according to claim 1, wherein the unsaturated metal
carboxylic acid salt is selected from among the group consisting of
zinc acrylate, zinc diacrylate, zinc methacrylate, zinc
dimethacrylate, and mixtures of these compounds.
7. The tread according to claim 6, wherein the unsaturated metal
carboxylic acid salt is zinc diacrylate ("ZDA") or zinc
dimethacrylate ("ZDMA").
8. The tread according to claim 1, wherein the amount of
unsaturated metal carboxylic acid salt is between 1 and 25 phr.
9. The tread according to claim 8, wherein the amount of
unsaturated metal carboxylic acid salt is within a range from 5 to
20 phr.
10. The tread according to claim 1, wherein the rubber composition
furthermore comprises a radical initiator.
11. The tread according to claim 10, wherein the radical initiator
is a heat-triggered radical initiator.
12. The tread according to claim 11, wherein the heat-triggered
radical is an organic peroxide.
13. The tread according to claim 12, wherein the amount of organic
peroxide is between 1 and 5 phr.
14. The tread according to claim 1, wherein the diene elastomer is
a butadiene/styrene copolymer (SBR).
15. The tread according to claim 14, wherein the SBR elastomer has
a styrene content of between 20% and 30% by weight, a content of
vinyl bonds of the butadiene fraction of between 15% and 65%, a
content of trans-1,4 bonds of between 20% and 75% and a glass
transition temperature of between -20.degree. C. and -55.degree.
C.
16. The tread according to claim 14, further comprising wherein the
SBR is used in a mixture with a polybutadiene having more than 90%
cis-1,4 bonds.
17. The tread according to claim 1, further comprising wherein the
reinforcing inorganic filler is used in a mixture with carbon black
present in an amount less than 20 phr.
18. The tread according to claim 17, wherein the carbon black is
present in an amount between 2 and 15 phr.
19. The tread according to claim 1, further comprising wherein the
tread comprises at least two different, radially superposed, rubber
compositions, the rubber composition comprising the unsaturated
metal carboxylic acid salt forming the radially outer part of this
tread.
20. The tread according to claim 1, wherein the tread comprises at
least two different, radially superposed, rubber compositions, the
rubber composition comprising the unsaturated metal carboxylic acid
salt forming the radially inner part of the tread.
21. The tread according to claim 1, wherein the tread is in the
vulcanized state.
22. A process for preparing a tire tread having, after curing and
mechanical running-in of the tire comprising it, a rigidity
gradient which increases radially from the surface towards the
inside of the tread, characterized in that it comprises the
following steps: incorporating in a diene elastomer, in a mixer,
during a first step referred to as "non-productive": more than 50
phr of a reinforcing inorganic filler; between 2 and 15 phr of a
coupling agent; and an unsaturated metal carboxylic acid salt; by
thermomechanically kneading the entire mixture, in one or more
stages, until a maximum temperature of between 130.degree. C. and
200.degree. C. is reached; cooling the entire mixture to a
temperature of less than 100.degree. C.; then incorporating, during
a second step referred to as "productive": a vulcanization system;
kneading the entire mixture until a maximum temperature less than
120.degree. C. is reached; and extruding or calendering the rubber
composition thus obtained, in the form of a tire tread.
23. The process according to claim 22, wherein the diene elastomer
is selected from among the group consisting of polybutadienes,
synthetic polyisoprenes, natural rubber, butadiene copolymers,
isoprene copolymers, and mixtures of these elastomers.
24. The process according to claim 22, wherein the reinforcing
inorganic filler is a siliceous or aluminous filler.
25. The process according to claim 24, wherein the reinforcing
inorganic filler is silica.
26. The process according to claim 22, wherein the quantity of
reinforcing inorganic filler is between 60 and 120 phr.
27. The process according to claim 22, wherein the unsaturated
metal carboxylic acid salt is selected from among the group
consisting of zinc acrylate, zinc diacrylate, zinc methacrylate,
zinc dimethacrylate, and mixtures of these compounds.
28. The process according to claim 27, wherein the unsaturated
metal carboxylic acid salt is zinc diacrylate ("ZDA") or zinc
dimethacrylate ("ZDMA").
29. The process according to claim 22, wherein the amount of
unsaturated metal carboxylic acid salt is between 1 and 25 phr.
30. The process according to claim 29, wherein the amount of
unsaturated metal carboxylic acid salt is within a range from 5 to
20 phr.
31. The process according to claim 22, further comprising wherein a
radical initiator is added during the productive step.
32. The process according to claim 31, wherein the radical
initiator is a heat-triggered radical initiator.
33. The process according to claim 32, wherein the heat-triggered
radical initiator is an organic peroxide.
34. The process according to claim 33, wherein the amount of
organic peroxide is between 1 and 5 phr.
35. A tire comprising a tread according to any one of claims 1 to
21.
36. The tire according to claim 35, wherein the tire is a
passenger-car tire, wherein the tread of said tire has, after
vulcanization and mechanical running-in of the tire, a rigidity
gradient which increases radially from the surface towards the
inside of the tread, the modulus ME10 (secant modulus at 10%
elongation) of said tread being between 2 and 8 MPa at the surface
of said tread, and between 8 and 16 MPa in the radially innermost
part of said tread.
37. The tire according to claim 36, wherein the tire is a summer
passenger-car tire, the modulus ME10 of the tread being between 5
and 8 MPa at the surface of said tread, and between 8 and 14 MPa in
the radially innermost part of said tread.
38. The tire according to claim 36, wherein the tire is a winter
passenger-car tire, the modulus ME10 of the tread being between 3
and 6 MPa at the surface of said tread, and between 6 and 12 MPa in
the radially innermost part of said tread.
Description
[0001] The present application is a continuation of International
Application No. PCT/EP03/01125, filed Feb. 5, 2003, published in
French with an English Abstract on Aug. 14, 2003 under PCT Article
21(2) as WO03/066352, which claims priority to French Patent
Application No. 02/01658, filed Feb. 7, 2002.
[0002] The present invention relates to treads for tires and to
rubber compositions used for the manufacture of such treads.
[0003] It relates more particularly to treads for tires having a
low rolling resistance, reinforced majoritarily by inorganic
fillers, these treads being intended in particular for tires fitted
on vehicles such as motorcycles, passenger cars, vans or heavy
vehicles.
[0004] Since fuel economies and the need to protect the environment
have become priorities, it has proved necessary to produce tires
having both reduced rolling resistance and high wear resistance.
This has been made possible due in particular to the discovery of
new rubber compositions reinforced with specific inorganic fillers
referred to as "reinforcing" fillers, which are capable of
rivalling conventional carbon black from the reinforcing point of
view, and furthermore offering these compositions a low hysteresis,
which is synonymous with lesser rolling resistance for the tire
treads comprising them. Such compositions based on reinforcing
inorganic fillers of the siliceous or aluminous type have for
example been described in patents or patent applications EP-A-0 501
227, EP-A-0 735 088, EP-A-0 810 258, EP-A-0 881 252, WO99/02590,
WO99/02601, WO99/02602, WO99/28376, WO00/05300, WO00/05301,
WO01/96442, WO02/30939, WO02/31041 and WO02/083782.
[0005] However, ideally, a tire tread must meet other technical
demands, some of which are contradictory, have in particular very
good grip both on dry ground and on wet, snow-covered or icy
ground, while offering the tire a very good level of road behavior
("handling") on an automobile, in particular high drift thrust (or
"cornering").
[0006] To improve the road behavior, it is known that greater
rigidity of the tread is desirable, this stiffening possibly being
obtained for example by increasing the amount of reinforcing filler
or by incorporating certain reinforcing resins into the rubber
compositions constituting these treads.
[0007] However, such stiffening of the tread, at the very least for
its surface part which is in contact with the ground during rolling
of the tire, is known to impair, most frequently in crippling
manner, the properties of grip on wet, snow-covered or icy
ground.
[0008] This is why, in order to meet these two contradictory
demands, namely road behavior and grip, it has essentially been
proposed hitherto to use composite treads (i.e., hybrid treads),
formed by two radially superposed layers ("cap-base structure") of
different rigidities, formed of two rubber compositions of
different formulations: the radially outer layer, in contact with
the road, is formed of the more flexible composition, in order to
meet the grip requirements; the radially inner layer is formed of
the more rigid composition, in order to meet the road behaviour
requirements.
[0009] Such a solution however has numerous disadvantages:
[0010] first of all, the manufacturing of a composite tread is by
definition more complex and therefore more costly than that of a
conventional tread, and requires in particular the use of complex
coextrusion machines;
[0011] during manufacturing, after cutting out the tread to the
correct dimensions once it has emerged from the extruder, it is
furthermore necessary to manage discarding of material of different
natures, which further substantially increases the production
costs;
[0012] finally, and this is not the least of the disadvantages,
once the radially outer (flexible) part of the tread has become
worn, it is the initially inner part of the tread which comes into
contact with the road: then, of course, one has the disadvantages
of an excessively rigid tread, with unsatisfactory performance from
the point of view of the technical compromise initially
intended.
[0013] Now, the Applicant has discovered during his research that a
specific rubber composition, based on a high amount of reinforcing
inorganic filler and an unsaturated metal carboxylic acid salt,
makes it possible, owing to an unexpected "auto-accommodation"
phenomenon, to obtain a tread having a true rigidity gradient,
radially increasing from the surface towards the inside of the
tread. This rigidity gradient is obtained not only simply and
economically, but also durably, thus making it possible to maintain
the compromise between grip and road behavior of the tires at a
very high level, throughout the life of the latter.
[0014] Consequently, a first subject of the invention relates to a
tire tread comprising a rubber composition based on at least
(phr=parts by weight per hundred parts of elastomer):
[0015] (i) a diene elastomer;
[0016] (ii) more than 50 phr of a reinforcing inorganic filler;
[0017] (iii) between 2 and 15 phr of a coupling agent;
[0018] (iv) an unsaturated metal carboxylic acid salt.
[0019] The subject of the invention is also the use of such a tread
for the manufacturing of new tires or the recapping of worn tires.
The tread according to the invention is particularly suited to
tires intended to be fitted on passenger vehicles, 4.times.4
vehicles (having 4 driving wheels), motorcycles, vans and heavy
vehicles (i.e., subway trains, buses, road transport machinery,
off-road vehicles).
[0020] The subject of the invention is also these tires themselves
when they comprise a tread according to the invention. It relates
in particular to tires of "winter" type intended for snow-covered
or icy roads.
[0021] Another subject of the invention is a process for preparing
a tire tread, having, after curing and mechanical running-in of the
tire comprising it, a rigidity gradient which increases radially
from the surface towards the inside of the tread, this process
being characterized in that it comprises the following steps:
[0022] incorporating in a diene elastomer, in a mixer, during a
first step referred to as "non-productive":
[0023] more than 50 phr of a reinforcing inorganic filler;
[0024] between 2 and 15 phr of a coupling agent;
[0025] an unsaturated metal carboxylic acid salt,
[0026] thermomechanically kneading the entire mixture, in one or
more stages, until a maximum temperature of between 130.degree. C.
and 200.degree. C. is reached;
[0027] cooling the entire mixture to a temperature of less than
100.degree. C.;
[0028] then incorporating, during a second step referred to as
"productive":
[0029] a vulcanization system, with which there is preferably
associated
[0030] a radical initiator;
[0031] kneading the entire mixture until a maximum temperature less
than 120.degree. C. is reached;
[0032] extruding or calendering the rubber composition thus
obtained, in the form of a tire tread.
[0033] The invention and its advantages will be readily understood
in the light of the description and the examples which follow.
[0034] I. Measurements and Tests Used
[0035] The treads and rubber compositions constituting these treads
are characterized or tested as indicated hereafter.
[0036] I-1. Shore A Hardness
[0037] The Shore A hardness of the compositions after curing is
assessed in accordance with ASTM Standard D 2240-86.
[0038] I-2. Tensile Tests
[0039] The tensile tests make it possible to determine the
elasticity stresses and the properties at break, after curing.
Unless indicated otherwise, they are effected in accordance with
French Standard NF T 46-002 of September 1988. The nominal secant
moduli (or apparent stresses, in MPa) at 10% elongation (ME10) are
measured in a second elongation (i.e., after a cycle of
accommodation to the amount of extension provided for the
measurement itself).
[0040] The nominal secant modulus is also measured at 10%
elongation, after an accommodation of 15% (i.e., an extension to
15% followed by relaxation to 0%) instead of 10% as previously for
the modulus ME10. This so-called "accommodated" modulus is referred
to as ME10AC. All these tensile measurements are effected under
normal conditions of temperature and humidity (23+2.degree. C. and
50+5% relative humidity, in accordance with French standard NF T
40-101 of December 1979).
[0041] I-3. Mechanical Accommodation
[0042] "Mechanical accommodation" is understood to mean simple
running-in of the tire by means of which its tread is placed in
contact with the ground during rolling, that is to say in working
conditions, for several tens of seconds or several minutes at the
most. This running-in operation may be carried out on an automatic
rolling machine or directly on an automobile; it may be effected in
various ways, for example by simply rolling in a straight line of
several tens or hundreds of meters, by longitudinal braking or
alternatively by drifting of the tire (bends), the important thing
being to start making the tread "work" under normal conditions of
use.
[0043] By way of example, such mechanical accommodation may be
achieved by what is called "standard" running-in consisting of
simple running in a straight line over a length of 400 meters at a
speed of 60 km/h, on a given automobile, without drifting or
cambering imposed on the tire, followed by moderate longitudinal
braking (braking distance from 30 to 40 meters) to stop the
vehicle. This standard running-in is furthermore effected under
normal conditions of pressure (those recommended by the
manufacturer of the vehicle used) and load (1 person only on board
the vehicle).
II. DETAILED DESCRIPTION OF THE INVENTION
[0044] The treads according to the invention are formed, at least
in part, of a rubber composition based on at least: (i) a (at least
one) diene elastomer; (ii) a minimum quantity (more than 50 hr) of
a (at least one) inorganic filler as reinforcing filler; (iii) a
(at least one) coupling agent (between 2 and 15 phr) providing the
bond between the reinforcing inorganic filler and this diene
elastomer; (iv) a (at least one) unsaturated metal carboxylic acid
salt with which may be associated (v) a (at least one) radical
initiator.
[0045] Of course, the expression composition "based on" is to be
understood to mean a composition comprising the mix and/or the
product of reaction in situ of the various constituents used, some
of these base constituents (for example, the coupling agent, the
metal carboxylic acid salt and the radical initiator if present)
being liable to, or intended to, react together, at least in part,
during the different phases of manufacturing of the treads, in
particular during the vulcanization or curing thereof.
[0046] In the present description, unless expressly indicated
otherwise, all the percentages (%) indicated are mass %.
[0047] II-1. Diene Elastomer
[0048] "Diene" elastomer or rubber is understood to mean,
generally, an elastomer resulting at least in part (i.e., a
homopolymer or a copolymer) from diene monomers (monomers bearing
two double carbon-carbon bonds, whether conjugated or not).
"Essentially unsaturated" diene elastomer is understood here to
mean a diene elastomer resulting at least in part from conjugated
diene monomers, having a content of members or units of diene
origin (conjugated dienes) which is greater than 15% (mol %). Thus,
for example, diene elastomers such as butyl rubbers or copolymers
of dienes and of alpha-olefins of the EPDM type do not fall within
this definition, and may on the contrary be described as
"essentially saturated" diene elastomers (low or very low content
of units of diene origin which is always less than 15%). Within the
category of "essentially unsaturated" diene elastomers, "highly
unsaturated" diene elastomer is understood to mean in particular a
diene elastomer having a content of units of diene origin
(conjugated dienes) which is greater than 50%.
[0049] These general definitions being given, the person skilled in
the art of tires will understand that the present invention is used
first and foremost with highly unsaturated diene elastomers, in
particular with:
[0050] (a)--any homopolymer obtained by polymerization of a
conjugated diene monomer having 4 to 12 carbon atoms;
[0051] (b)--any copolymer obtained by copolymerisation of one or
more conjugated dienes with each other or with one or more
vinyl-aromatic compounds having 8 to 20 carbon atoms.
[0052] Suitable conjugated dienes are, in particular,
1,3-butadiene, 2-methyl-1,3-butadiene,
2,3-di(C.sub.1-C.sub.5)alkyl-1,3-butadienes such as, for instance,
2,3-dimethyl-1,3-butadiene, 2,3-diethyl-1,3-butadiene,
2-methyl-3-ethyl-1,3-butadiene, 2-methyl-3-isopropyl-1,3-butadiene,
an aryl-1,3-butadiene, 1,3-pentadiene and 2,4-hexadiene. Suitable
vinyl-aromatic compounds are, for example, styrene, ortho-, meta-
and para-methylstyrene, the commercial mixture "vinyltoluene",
para-tert butylstyrene, methoxystyrenes, chlorostyrenes,
vinylmesitylene, divinylbenzene and vinylnaphthalene.
[0053] The copolymers may contain between 99% and 20% by weight of
diene units and between 1% and 80% by weight of vinyl-aromatic
units. The elastomers may have any microstructure, which is a
function of the polymerization conditions used, in particular of
the presence or absence of a modifying and/or randomizing agent and
the quantities of modifying and/or randomising agent used. The
elastomers may for example be block, statistical, sequential or
microsequential elastomers, and may be prepared in dispersion or in
solution; they may be coupled and/or starred or alternatively
functionalized with a coupling and/or starring or functionalizing
agent.
[0054] Preferred are polybutadienes, and in particular those having
a content of 1,2-units of between 4% and 80%, or those having a
content of cis-1,4 of more than 80%, polyisoprenes,
butadiene/styrene copolymers, and in particular those having a
styrene content of between 5% and 50% by weight and, more
particularly, between 20% and 40%, a content of 1,2-bonds of the
butadiene fraction of between 4% and 65%, and a content of
trans-1,4 bonds of between 20% and 80%, butadiene/isoprene
copolymers and in particular those having an isoprene content of
between 5% and 90% by weight and a glass transition temperature
("Tg"-measured in accordance with ASTM Standard D3418-82) of
-40.degree. C. to -80.degree. C., isoprene/styrene copolymers and
in particular those having a styrene content of between 5% and 50%
by weight and a Tg of between -25.degree. C. and -50.degree. C. In
the case of butadiene/styrene/isoprene copolymers, those which are
suitable are in particular those having a styrene content of
between 5% and 50% by weight and, more particularly, between 10%
and 40%, an isoprene content of between 15% and 60% by weight, and
more particularly between 20% and 50%, a butadiene content of
between 5% and 50% by weight, and more particularly between 20% and
40%, a content of 1,2-units of the butadiene fraction of between 4%
and 85%, a content of trans-1,4 units of the butadiene fraction of
between 6% and 80%, a content of 1,2- plus 3,4-units of the
isoprene fraction of between 5% and 70%, and a content of trans-1,4
units of the isoprene fraction of between 10% and 50%, and more
generally any butadiene/styrene/isoprene copolymer having a Tg of
between -20.degree. C. and -70.degree. C.
[0055] In summary, particularly preferably, the diene elastomer of
the composition used in the tread according to the invention is
selected from the group of highly unsaturated diene elastomers
constituted by polybutadienes (BR), synthetic polyisoprenes (IR),
natural rubber (NR), butadiene copolymers, isoprene copolymers and
mixtures of these elastomers. Such copolymers are more preferably
selected from the group which consists of butadiene/styrene
copolymers (SBR), butadiene/isoprene copolymers (BIR),
isoprene/styrene copolymers (SIR), isoprene/butadiene/styrene
copolymers (SBIR) and mixtures of such copolymers.
[0056] The tread according to the invention is preferably intended
for a passenger-car tire. In such a case, the diene elastomer is
preferably an SBR copolymer, in particular an SBR prepared in
solution, preferably used in a mixture with a polybutadiene; more
preferably, the SBR has a content of styrene of between 20% and 30%
by weight, a content of vinyl bonds of the butadiene fraction of
between 15% and 65%, a content of trans-1,4 bonds of between 15%
and 75% and a Tg of between -20.degree. C. and -55.degree. C., and
the polybutadiene has more than 90% cis-1,4 bonds.
[0057] The compositions of the treads of the invention may contain
a single diene elastomer or a mixture of several diene elastomers,
the diene elastomer(s) possibly being used in association with any
type of synthetic elastomer other than a diene one, or even with
polymers other than elastomers, for example thermoplastic
polymers.
[0058] II-2. Reinforcing Inorganic Filler
[0059] "Reinforcing inorganic filler" is to be understood in known
manner to mean any inorganic or mineral filler, whatever its colour
and its origin (natural or synthetic), also referred to as "white"
filler or sometimes "clear" filler, in contrast to carbon black,
which is capable, on its own, without any other means than an
intermediate coupling agent, of reinforcing a rubber composition
intended for the manufacture of a tire tread, in other words which
is capable of replacing a conventional tire-grade carbon black (for
treads) in its reinforcement function.
[0060] Preferably, the reinforcing inorganic filler is a filler of
the siliceous (for example silica) or aluminous (for example
alumina) type, or a mixture of these two types of fillers.
[0061] The silica (SiO.sub.2) used may be any reinforcing silica
known to the person skilled in the art, in particular any
precipitated or fumed silica having a BET surface area and a CTAB
specific surface area both of which are less than 450 m.sup.2/g,
preferably from 30 to 400 m.sup.2/g. Highly dispersible
precipitated silicas (referred to as "HDS") are preferred, in
particular when the invention is used for the manufacture of tires
having a low rolling resistance; "highly dispersible silica" is
understood in known manner to mean any silica having a substantial
ability to disagglomerate and to disperse in an elastomeric matrix,
which can be observed in known manner by electron or optical
microscopy on thin sections. As examples of such preferred highly
dispersible silicas, mention may be made of the silicas Ultrasil
7000 and Ultrasil 7005 from Degussa, the silicas Zeosil 1165MP,
1135MP and 1115MP from Rhodia, the silica Hi-Sil EZ150G from PPG,
the silicas Zeopol 8715, 8745 and 8755 from Huber, and treated
precipitated silicas such as, for example, the aluminium-"doped"
silicas described in the aforementioned application EP-A-0 735
088.
[0062] The reinforcing alumina (Al.sub.2O.sub.3) preferably used is
a highly dispersible alumina having a BET surface area from 30 to
400 m.sup.2/g, more preferably between 60 and 250 m.sup.2/g, an
average particle size at most equal to 500 nm, more preferably at
most equal to 200 nm, as described in the aforementioned
application EP-A-0 810 258. Examples of such reinforcing aluminas
are in particular the aluminas "Baikalox", "A125" or "CR125" (from
Baikowski), "APA-100RDX" (from Condea), "Aluminoxid C" (from
Degussa) or "AKP-G015" (Sumitomo Chemicals). The invention can also
be implemented by using as reinforcing inorganic filler the
specific aluminium (oxide-)hydroxides such as described in
WO99/28376.
[0063] The physical state in which the reinforcing inorganic filler
may be present is immaterial, whether it be in the form of a
powder, microbeads, granules, pellets, balls or any other
appropriate densified form.
[0064] Of course, "reinforcing inorganic filler" is also understood
to mean mixtures of different reinforcing inorganic fillers, in
particular of highly dispersible siliceous and/or aluminous fillers
such as described above.
[0065] When the treads of the invention are intended for tires of
low rolling resistance, the reinforcing inorganic filler used, in
particular if it is silica, preferably has a BET surface area of
between 60 and 250 m.sup.2/g, more preferably between 80 and 230
m.sup.2/g.
[0066] The inorganic filler used as reinforcing filler must be
present in a high amount, greater than 50 phr, preferably greater
than 60 phr, which is one of the essential characteristics of the
invention, this reinforcing inorganic filler possibly constituting
all or the majority of the total reinforcing filler, in this latter
case associated for example with a minority quantity of carbon
black (preferably less than 20 phr, more preferably less than 15
phr).
[0067] The person skilled in the art will readily understand that
the optimum amount will differ according to the nature of the
reinforcing inorganic filler used and the type of tire in question,
for example tire for motorcycles, for passenger vehicles or
alternatively for utility vehicles such as vans or heavy vehicles.
Preferably, the amount of reinforcing inorganic filler is between
60 and 120 phr, more preferably still between 70 and 110 phr
approximately, for example within a range from 80 to 105 phr in the
specific case of treads for passenger-vehicle tires.
[0068] Preferably, in the tread according to the invention, the
reinforcing inorganic filler constitutes more than 80% by weight of
the total reinforcing filler, more preferably more than 90% by
weight (or even all) of this total reinforcing filler. However,
without significantly affecting the technical effect desired, a
small quantity of carbon black, preferably less than 20%, more
preferably still less than 10% by weight relative to the quantity
of total reinforcing filler, may be used.
[0069] The carbon black, if used, is preferably present in an
amount of between 2 and 15 phr, more preferably between 4 and 12
phr. It can be used in particular as a simple black pigmentation
agent, or alternatively to protect the tread from different sources
of atmospheric ageing such as ozone, oxidation or UV radiation. On
the other hand, it is known that certain rubber-making additives,
in particular certain coupling agents, are available in a form
supported by carbon black, the use of such additives therefore
involving the incorporation, in a small proportion, of carbon
black. Suitable carbon blacks are any carbon blacks, in particular
the blacks of the type HAF, ISAF and SAF, which are conventionally
used in tires, and particularly in treads for these tires; as
non-limitative examples of such blacks, mention may be made of the
blacks N115, N134, N234, N339, N347 and N375.
[0070] In the present specification, the BET specific surface area
is determined in known manner by adsorption of gas using the method
of Brunauer-Emmett-Teller described in "The Journal of the American
Chemical Society" Vol. 60, page 309, February 1938, more precisely
in accordance with French Standard NF ISO 9277 of December 1996
[multipoint volumetric method (5 points)--gas: nitrogen--degassing:
1 hour at 160.degree. C.--range of relative pressure p/po: 0.05 to
0.17]. The CTAB specific surface area is the external surface area
determined in accordance with French Standard NF T 45-007 of
November 1987 (method B).
[0071] Finally, as filler equivalent to such a reinforcing
inorganic filler, there could be used a reinforcing filler of
organic type, in particular a carbon black, coated at least in part
with an inorganic layer (for example, a layer of silica), which for
its part requires the use of a coupling agent to provide the
connection to the elastomer.
[0072] II-3. Coupling Agent
[0073] In known manner, in the presence of a reinforcing inorganic
filler, it is necessary to use a coupling agent or bonding agent,
the function of which is to provide a sufficient chemical and/or
physical connection between the inorganic filler (surface of its
particles) and the diene elastomer.
[0074] Such a coupling agent, which is consequently at least
bifunctional, has, for example, the simplified general formula
"Y-T-X", in which:
[0075] Y represents a functional group ("Y" function) which is
capable of bonding physically and/or chemically with the inorganic
filler, such a bond being able to be established, for example,
between a silicon atom of the coupling agent and the surface
hydroxyl (OH) groups of the inorganic filler (for example, surface
silanols in the case of silica);
[0076] X represents a functional group ("X" function) which is
capable of bonding physically and/or chemically with the diene
elastomer, for example by means of a sulphur atom;
[0077] T represents a divalent group making it possible to link Y
and X.
[0078] The coupling agents must particularly not be confused with
simple agents for coating the inorganic filler which, in known
manner, may comprise the function Y which is active with respect to
the inorganic filler but are devoid of the function X which is
active with respect to the elastomer.
[0079] (Silica/diene elastomer) coupling agents, of variable
effectiveness, have been described in a very large number of
documents and are well-known to the person skilled in the art. Any
coupling agent likely to ensure, in the diene rubber compositions
usable for the manufacturing of tire treads, the effective bonding
between a reinforcing inorganic filler such as silica and a diene
elastomer, in particular organosilanes or polyfunctional
polyorganosiloxanes bearing the functions X and Y, may be used.
[0080] In particular polysulphurized silanes, which are referred to
as "symmetrical" or "asymmetrical" depending on their specific
structure, are used, such as those described for example in the
patents or patent applications FR 2 149 339, FR 2 206 330, U.S.
Pat. No. 3,842,111, U.S. Pat. No. 3,873,489, U.S. Pat. No.
3,978,103, U.S. Pat. No. 3,997,581, U.S. Pat. No. 4,002,594, U.S.
Pat. No. 4,072,701, U.S. Pat. No. 4,129,585, U.S. Pat. No.
5,580,919, U.S. Pat. No. 5,583,245, U.S. Pat. No. 5,650,457, U.S.
Pat. No. 5,663,358, U.S. Pat. No. 5,663,395, U.S. Pat. No.
5,663,396, U.S. Pat. No. 5,674,932, U.S. Pat. No. 5,675,014, U.S.
Pat. No. 5,684,171, U.S. Pat. No. 5,684,172, U.S. Pat. No.
5,696,197, U.S. Pat. No. 5,708,053, U.S. Pat. No. 5,892,085, EP 1
043 357 or WO02/083782.
[0081] Particularly suitable for implementing the invention,
without the definition below being limitative, are what are called
"symmetrical" polysulphurized silanes which satisfy the following
general formula (I):
Z-A-S.sub.n-A-Z, in which: (I)
[0082] n is an integer from 2 to 8 (preferably from 2 to 5);
[0083] A is a divalent hydrocarbon radical (preferably
C.sub.1-C.sub.18 alkylene groups or C.sub.6-C.sub.12 arylene
groups, more particularly C.sub.1-C.sub.10 alkylenes, notably
C.sub.1-C.sub.4 alkylenes, in particular propylene);
[0084] Z corresponds to one of the formulae below: 1
[0085] in which:
[0086] the radicals R.sup.1, which may or may not be substituted,
and may be identical or different, represent a C.sub.1-C.sub.18
alkyl group, a C.sub.5-C.sub.18 cycloalkyl group or a
C.sub.6-C.sub.18 aryl group, (preferably C.sub.1-C.sub.6 alkyl
groups, cyclohexyl or phenyl, in particular C.sub.1-C.sub.4 alkyl
groups, more particularly methyl and/or ethyl).
[0087] the radicals R.sup.2, which may or may not be substituted,
and may be identical or different, represent a C.sub.1-C.sub.18
alkoxyl group or a C.sub.5-C.sub.18 cycloalkoxyl group (preferably
a group selected from among C.sub.1-C.sub.8 alkoxyls and
C.sub.5-C.sub.8 cycloalkoxyls, more preferably still a group
selected from among C.sub.1-C.sub.4 alkoxyls, in particular
methoxyl and/or ethoxyl).
[0088] In the case of a mixture of polysulphurized alkoxysilanes in
accordance with Formula (I) above, in particular conventional,
commercially available, mixtures, the average value of the "n"s is
a fractional number, preferably between 2 and 5, more preferably
close to 4. However, the invention may also be implemented
advantageously for example with disulphurized alkoxysilanes
(n=2).
[0089] As examples of polysulphurized silanes, mention will be made
more particularly of the polysulphides (in particular disulphides,
trisulphides or tetrasulphides) of
bis-((C.sub.1-C.sub.4)alkoxyl-(C.sub.1-
-C.sub.4)alkylsilyl(C.sub.1-C.sub.4)alkyl), such as for example the
polysulphides of bis(3-trimethoxysilylpropyl) or of
bis(3-triethoxysilylpropyl). Of these compounds, in particular
bis(3-triethoxysilylpropyl) tetrasulphide, abbreviated TESPT, of
the formula
[(C.sub.2H.sub.5O).sub.3Si(CH.sub.2).sub.3S.sub.2].sub.2, or
bis(triethoxysilylpropyl) disulphide, abbreviated TESPD, of the
formula [(C.sub.2H.sub.5O).sub.3Si(CH.sub.2).sub.3S].sub.2, are
used.
[0090] TESPD is sold, for example, by Degussa under the name Si75
(in the form of a mixture of disulphide--75% by weight--and of
polysulphides), or alternatively by Witco under the name Silquest
A1589. TESPT is sold, for example, by Degussa under the name Si69
(or X50S when it is supported to 50% by weight on carbon black), or
alternatively by Osi Specialties under the name Silquest Al 289 (in
both cases, a commercial mixture of polysulphides having an average
value of n which is close to 4).
[0091] Mention will also be made as advantageous coupling agent of
the polysulphides (in particular disulphides, trisulphides or
tetrasulphides) of
bis-(mono(C.sub.1-C.sub.4)alkoxyl-di(C.sub.1-C.sub.4)alkylsilylpropyl)-
, more particularly bis-monoethoxydimethylsilylpropyl tetrasulphide
as described in the aforementioned application WO02/083782.
[0092] As examples of coupling agents other than the aforementioned
polysulphurized alkoxysilanes, mention will be made in particular
of the bifunctional polyorganosiloxanes such as described for
example in the aforementioned applications WO99/02602 or
WO01/96442, or alternatively the hydroxysilane polysulphides such
as described in the aforementioned applications WO02/30939 and
WO02/31041.
[0093] In the treads according to the invention, the content of
coupling agent is preferably between 4 and 12 phr, more preferably
between 3 and 8 phr. However, it is generally desirable to use as
little as possible thereof. Relative to the weight of reinforcing
inorganic filler, the amount of coupling agent typically represents
between 0.5 and 15% by weight relative to the quantity of
reinforcing inorganic filler. In the case for example of tire
treads for passenger vehicles, the coupling agent is used in a
preferred amount of less than 12%, or even less than 10% by weight
relative to this quantity of reinforcing inorganic filler.
[0094] The coupling agent could be grafted beforehand (via the "X"
function) on to the diene elastomer of the composition of the
invention, the elastomer thus functionalised or "precoupled" then
comprising the free "Y" function for the reinforcing inorganic
filler. The coupling agent could also be grafted beforehand (via
the "Y" function) on to the reinforcing inorganic filler, the
filler thus "precoupled" then being able to be bonded to the diene
elastomer by means of the free "X" function. However, it is
preferred, in particular for reasons of better processing of the
compositions in the uncured state, to use the coupling agent either
grafted onto the reinforcing inorganic filler, or in the free
(i.e., non-grafted) state.
[0095] There may possibly be associated with the coupling agent an
appropriate "coupling activator", that is to say, a body (single
compound or association of compounds) which, when mixed with this
coupling agent, increases the effectiveness of the latter. Coupling
activators for polysulphurized alkoxysilanes have for example been
described in the aforementioned international applications
WO00/05300 and WO00/05301, consisting of the association of a
substituted guanidine, in particular N,N'-diphenylguanidine
(abbreviated to "DPG"), with an enamine or a zinc dithiophosphate.
The presence of these coupling activators will make it possible,
for example, to keep the amount of coupling agent at a preferred
level of less than 10%, or even less than 8% by weight relative to
the quantity of reinforcing inorganic filler, or alternatively to
reduce the amount of reinforcing inorganic filler owing to the
improved coupling with the diene elastomer.
[0096] II-4. Unsaturated Metal Carboxylic Acid Salt
[0097] The rubber compositions of the treads of the invention
contain at least an unsaturated metal carboxylic acid salt intended
to form in situ, after curing (vulcanization) of the tread, a
three-dimensional resin lattice which is superposed and
interpenetrates with the (inorganic filler/elastomer) lattice on
one hand, and with the (elastomer/sulphur) lattice on the other
hand (if the cross-linking agent is sulphur).
[0098] Unsaturated metal carboxylic acid salts, in particular zinc
(meth)acrylates, whether or not associated with a radical initiator
such as an organic peroxide, are well-known to the person skilled
in the art for their ability to form, by polycondensation of their
carboxylic double bonds, a three-dimensional reinforcing resin
lattice (hereafter "acrylate lattice" in the case of an acid of
acrylic type) upon a curing operation of the rubber matrix. They
have been used in rubber compositions, in particular for tires, in
very variable amounts which may vary for example between 0.1 and 50
phr, for applications as varied as adhesion or reinforcement (see
for example EP-A-0 420 449, EP-A-0 552 620, GB-A-2 042 553, U.S.
Pat. No. 3,344,105, US-A-3,522,223, U.S. Pat. No. 3,823,122, U.S.
Pat. No. 4,082,288, U.S. Pat. No. 4,191,671, U.S. Pat. No.
4,495,326, US-A-4,529,770, U.S. Pat. No. 4,529,770, U.S. Pat. No.
4,720,526, U.S. Pat. No. 4,987,192, U.S. Pat. No. 5,217,807,
US-A-6,153,686, U.S. Pat. No. 6,051,653, U.S. Pat. No. 6,251,977,
Japanese applications published under the Nos. JP1975/154386,
JP1993/051491, JP1994/278357, JP1994/287358, JP1996/134270 and JP
1996/269241).
[0099] However, as far as the Applicant is aware, no document of
the prior art describes the use in a tire tread of unsaturated
metal carboxylic acid salts, in particular of zinc (meth)acrylates,
whether or not associated with a radical initiator such as an
organic peroxide, in combination with such a high amount (more than
50 phr, preferably more than 60 phr) of a reinforcing inorganic
filler such as silica. The knowledge of the person skilled in the
art, acquired essentially in the field of rubber compositions
filled majoritarily with carbon black, on the contrary pointed him
away from such a use, owing to a stiffening ability, for this type
of metal salts, which is deemed a great disadvantage with regard to
the grip properties mentioned above.
[0100] The unsaturated carboxylic acid preferably belongs to the
group consisting of acrylic acid, ethacrylic acid, methacrylic
acid, cinnamic acid, crotonic acid, maleic acid, fumaric acid,
itaconic acid and mixtures of these acids. More particularly
acrylic acid or methacrylic acid are used.
[0101] The metal of the metal salt is preferably selected from the
preferred group constituted by Al, Ca, Mg, Cr, Mn, Fe, Co, Ni, Zn,
more preferably from among Al, Mg and Zn. Even more preferably,
zinc is used.
[0102] Zinc acrylic acid salts which are usable have for example
been described in "New metallic coagents for curing elastomers",
Sartomer Application Bulletin, April 1998 (Sartomer Co., Inc.). The
metal salt preferably used is a zinc (di)(meth)acrylate, that is to
say a salt selected from among zinc acrylate, zinc diacrylate, zinc
methacrylate, zinc dimethacrylate and -mixtures of these acrylates.
More preferably, zinc diacrylate (hereafter, abbreviated to "ZDA")
or zinc dimethacrylate (hereafter, abbreviated to "ZDMA") are
used.
[0103] The unsaturated metal carboxylic acid salt must be able to
disperse perfectly in the rubber matrix, at the same time as the
reinforcing inorganic filler and its coupling agent. By way of
examples of commercial products which are usable, mention will be
made for example of those from Sartomer sold under the names "Saret
633" (ZDA) or "Saret 634" (ZDMA).
[0104] The quantity of metal salt may vary to a great extent as a
function of the nature of the diene elastomer used and the quantity
of reinforcing inorganic filler, within a range of between 0.1 and
50 phr. This quantity is more preferably between 1 and 25 phr.
Below the minima indicated, the technical effect desired risks
being inadequate, whereas beyond the maxima indicated there are
risks of excessive stiffening and excessive compromising of the
hysteresis. A quantity within a range from 5 to 20 phr has proved
well suited, in particular in the case of treads for
passenger-vehicle tires.
[0105] II-5. Radical Initiator
[0106] With the unsaturated metal carboxylic acid salt previously
described, there is preferably associated a free-radical generator
or radical initiator, commonly referred to as "co-cross-linking
agent", which in known manner makes it possible to activate the
formation of the acrylate resin lattice due to the generation in
situ of free radicals, following energy activation.
[0107] Advantageously an initiator of the heat-triggered type is
used, that is to say that the supply of energy for creating the
free radicals is in heat form. Preferably a radical initiator is
selected which has a decomposition temperature of less than
180.degree. C., more preferably less than 160.degree. C., such
ranges of temperatures making it possible to benefit fully from the
activation effect, during the manufacture (kneading) of the rubber
compositions.
[0108] This heat-triggered radical initiator is selected from the
preferred group consisting of peroxides, hydroperoxides, azido
compounds, bis(azo) compounds and mixtures of these compounds, more
preferably from among the group consisting of (hydro)peroxides,
bis(azo) compounds and mixtures of these compounds.
[0109] By way of examples, mention will be made in particular of
benzoyl peroxide, acetyl peroxide, lauryl peroxide, cumyl peroxide,
tert.-butyl peroxide, tert.-butyl peracetate, tert.-butyl
hydroperoxide, cumene hydroperoxide, tert.-butyl cumyl peroxide,
2,5-dimethyl-2,5-bis(tert.-but- yl)-3-hexyne peroxide, 1,3-bis
(tert.-butyl-isopropyl) benzene peroxide, 2,4-dichlorobenzoyl
peroxide, tert.-butyl perbenzoate,
1,1-bis(tert.-butyl)3,3,5-trimethylcyclohexane peroxide,
1,1'-azobis(isobutyronitrile), 1,1'-azobis(secpentylnitrile) or
1,1'-azobis(cyclohexanecarbonitrile).
[0110] The quantity of radical initiator is between 0.05 and 10
phr, preferably between 0.1 and 5 phr. Below the minima indicated,
the activation effect risks being inadequate, whereas beyond the
maxima indicated there are the risks of compromising the processing
in the uncured state (scorching or premature cross-linking) or of
excessive stiffening of the compositions. For all these reasons, a
quantity within a range from 0.2 to 2.5 phr is more preferably
selected.
[0111] The amount of radical initiator, within the ranges indicated
above, is preferably adjusted so as to represent less than 30%,
more preferably less than 20% by weight relative to the quantity of
metal carboxylic acid salt, for example between 1% and 10%.
[0112] II-6. Various Additives
[0113] Of course, the rubber compositions of the treads according
to the invention also comprise all or part of the additives usually
used in sulphur-cross-linkable diene rubber compositions intended
for the manufacturing of treads, such as, for example,
plasticizers, pigments, protective agents of the type antioxidants,
antiozonants, a cross-linking system based either on sulphur or on
sulphur and/or peroxide and/or bismaleimide donors, vulcanization
accelerators, vulcanization activators or extender oils. There may
also be associated with the reinforcing inorganic filler, if
necessary, a conventional non-reinforcing white filler, such as for
example particles of clay, bentonite, talc, chalk, kaolin or
titanium oxides.
[0114] The rubber compositions of the treads of the invention may
also contain, in addition to the coupling agents, agents for
coating the reinforcing inorganic filler (comprising for example
the single function Y), or more generally processing aids liable,
in known manner, owing to an improvement in the dispersion of the
inorganic filler in the rubber matrix and to a reduction in the
viscosity of the compositions, to improve their processability in
the uncured state, these agents, used in a preferred amount of
between 0.5 and 3 phr, being, for example, alkylalkoxysilanes (in
particular alkyltriethoxysilanes), polyols, polyethers (for example
polyethylene glycols), primary, secondary or tertiary amines,
hydroxylated or hydrolysable polyorganosiloxanes, for example
.alpha.,.omega.-dihydroxypolyorganosiloxanes (in particular
.alpha.,.omega.-dihydroxypolydimethylsiloxanes).
[0115] II-7. Manufacturing of the Treads
[0116] The rubber compositions of the treads of the invention are
manufactured in suitable mixers, using two successive preparation
phases in accordance with a general process well-known to the
person skilled in the art: a first phase of thermomechanical
working or kneading (sometimes referred to as "non-productive"
phase) at high temperature, up to a maximum temperature of between
130.degree. C. and 200.degree. C., preferably between 145.degree.
C. and 185.degree. C., followed by a second phase of mechanical
working (sometimes referred to as "productive" phase) at lower
temperature, typically less than 120.degree. C., for example
between 60.degree. C. and 100.degree. C., during which finishing
phase the cross-linking or vulcanization system is
incorporated.
[0117] The process according to the invention, for preparing a tire
tread having, after curing and mechanical running-in of the tire
for which it is intended, a rigidity gradient radially increasing
from the surface towards the inside of the tread, comprises the
following steps:
[0118] incorporating in a diene elastomer, in a mixer, during a
first step referred to as "non-productive":
[0119] more than 50 phr of a reinforcing inorganic filler;
[0120] between 2 and 15 phr of a coupling agent;
[0121] an unsaturated metal carboxylic acid salt;
thermomechanically kneading the entire mixture, in one or more
stages, until a maximum temperature of between 130.degree. C. and
200.degree. C. is reached;
[0122] cooling the entire mixture to a temperature of less than
100.degree. C.;
[0123] then incorporating, during a second step referred to as
"productive":
[0124] a vulcanization system, with which there is preferably
associated
[0125] a radical initiator;
[0126] kneading the entire mixture until a maximum temperature less
than 120.degree. C. is reached;
[0127] extruding or calendering the rubber composition thus
obtained, in the form of a tire tread.
[0128] According to a preferred embodiment, all the base
constituents of the compositions of the treads according to the
invention, with the exception of the vulcanization system and any
radical initiator, namely the reinforcing inorganic filler, the
coupling agent and the metal carboxylic acid salt, are incorporated
intimately by kneading in the diene elastomer during the first,
so-called non-productive, phase, that is to say that at least these
different base constituents are introduced into the mixer and are
kneaded thermomechanically, in one or more stages, until the
maximum temperature of between 130.degree. C. and 200.degree. C.,
preferably between 145.degree. C. and 185.degree. C., is reached.
However, all or part of the metal carboxylic acid salt could also
be incorporated during the productive phase.
[0129] By way of example, the first (non-productive) phase is
effected in a single thermomechanical step during which all the
necessary constituents, any additional coating agents or processing
agents and various other additives, with the exception of the
vulcanization system and the radical initiator, are introduced into
a suitable mixer, such as a conventional internal mixer. A second
stage of thermomechanical working could possibly be added, in this
internal mixer, for example after an intermediate cooling stage
(preferably to a temperature of less than 100.degree. C.), with the
aim of making the compositions undergo complementary heat
treatment, in particular in order to improve the dispersion, in the
elastomeric matrix, of the reinforcing inorganic filler, the
coupling agent and the metal carboxylic acid salt.
[0130] After cooling the mixture thus obtained at the end of the
first, non-productive, phase, the vulcanization system and the
radical initiator are then incorporated at low temperature, in an
external mixer such as an open mill. The entire mixture is then
mixed (productive phase) for several minutes, for example between 5
and 15 minutes.
[0131] The vulcanization system proper is preferably based on
sulphur and a primary vulcanization accelerator, in particular an
accelerator of the sulphenamide type. To this vulcanization system
there are added, incorporated during the first, non-productive,
phase and/or during the productive phase, various known secondary
accelerators or vulcanization activators such as zinc oxide,
stearic acid, guanidine derivatives (in particular
diphenylguanidine). The amount of sulphur is preferably between 0.5
and 3.0 phr, and the amount of the primary accelerator is
preferably between 0.5 and 5.0 phr.
[0132] The final composition thus obtained is then calendered, for
example in the form of a film or a sheet, in particular for
characterization in the laboratory, or alternatively extruded in
the form of a rubber profiled element usable directly as a tire
tread.
[0133] The vulcanization (or curing) is carried out in known manner
at a temperature generally between 130.degree. C. and 200.degree.
C., for a sufficient time which may vary, for example, between 5
and 90 minutes, depending, in particular, on the curing
temperature, the vulcanization system adopted and the vulcanization
kinetics of the composition in question, and the size of the tire
in question.
[0134] To summarize, in the process according to the invention, in
accordance with all the particulars given previously, preferably at
least one, more preferably all, of the following characteristics
are satisfied:
[0135] the amount of reinforcing inorganic filler is between 60 and
120 phr;
[0136] the amount of coupling agent is between 4 and 12 phr;
[0137] the amount of metal salt is between 1 and 25 phr;
[0138] the amount of radical initiator is between 0.1 and 5
phr;
[0139] the maximum thermomechanical kneading temperature is between
145.degree. C. and 180.degree. C.;
[0140] the reinforcing inorganic filler is a siliceous or aluminous
filler;
[0141] the amount of carbon black is less than 20 phr, preferably
of between 2 and 15 phr;
[0142] the at least bifunctional coupling agent is an organosilane
or a polyorganosiloxane;
[0143] the metal carboxylic acid salt is selected from the group
consisting of zinc acrylate, zinc diacrylate, zinc methacrylate,
zinc dimethacrylate and mixtures of these acrylates;
[0144] the radical initiator is a heat-triggered radical
initiator;
[0145] the quantity of radical initiator represents less than 30%
relative to the weight of metal salt;
[0146] the diene elastomer is a butadiene/styrene copolymer (SBR),
preferably used in a mixture with a polybutadiene;
[0147] the reinforcing inorganic filler represents more than 80% by
weight of the total reinforcing filler.
[0148] More preferably, in this process, at least one, even more
preferably all, of the following characteristics are satisfied:
[0149] the amount of inorganic filler is between 70 and 110
phr;
[0150] the amount of coupling agent is between 3 and 8 phr;
[0151] the amount of metal salt lies within a range from 5 to 20
phr;
[0152] the amount of radical initiator lies within a range from 0.2
to 2.5 phr;
[0153] the reinforcing inorganic filler is silica;
[0154] the amount of carbon black is less than 15 phr, preferably
of between 4 and 12 phr;
[0155] the coupling agent is a bis-(C.sub.1-C.sub.4)silylpropyl
polysulphide;
[0156] the metal salt is zinc diacrylate (ZDA) or zinc
dimethacrylate (ZDMA), more preferably still ZDMA;
[0157] the radical initiator is an organic peroxide;
[0158] the quantity of radical initiator represents less than 20%,
preferably between 1% and 10%, relative to the weight of metal
salt;
[0159] the diene elastomer is an SBR prepared in solution used in a
mixture with a polybutadiene having more than 90% (mol) cis-1,4
bonds;
[0160] the reinforcing inorganic filler represents more than 90% by
weight of the total reinforcing filler.
[0161] Of course, the invention relates to the treads previously
described, both in the uncured state (i.e., before curing) and in
the cured state (i.e., after cross-linking or vulcanization).
[0162] II-8. Properties of the Treads
[0163] It can easily be confirmed that the combined presence of a
large amount of reinforcing filler, whatever the type of filler
used (carbon black or inorganic filler), and of unsaturated metal
carboxylic acid salt such as ZDA or ZDMA is accompanied, after
curing of the compositions, by a great increase in the moduli at
low deformation (the values of ME10 possibly being, for example,
doubled) and in the Shore A hardness (increased for example by 10
to 20%).
[0164] Such an increase in rigidity, which was expected, admittedly
allowed the person skilled in the art to predict, for tires mounted
on automobiles the treads of which were constituted by such
compositions, an improvement in road behavior owing to increased
stiffening and therefore increased drift thrust, but also and above
all an extremely disadvantageous drop in the grip performance on
wet, snow-covered or icy ground.
[0165] Such a drop in the grip performance has in fact been
observed for comparable treads filled with carbon black, but not in
the case of treads according to the invention: the latter,
surprisingly, benefit from the improved road behavior without
adversely affecting the grip performance above.
[0166] It must of necessity be deduced from this that the resin
lattice, in the rubber compositions of the treads, is "expressed"
differently depending on whether these compositions are filled
conventionally with carbon black, or on the contrary with a
reinforcing inorganic filler such as silica, in the high amounts
advocated.
[0167] Complementary tests have revealed an unexpected property for
the treads according to the invention, which a posteriori explains
the above differences in behavior.
[0168] These treads, owing to an unexpected phenomenon of
auto-accommodation after rolling, have a greatly marked rigidity
gradient in the radial direction, this rigidity increasing radially
from the surface towards the inside of the tread in a continuous
profile, over the entire thickness of the tread or at the very
least of the portion of hybrid tread, if applicable, comprising the
composition reinforced with inorganic filler and the unsaturated
metal carboxylic acid salt according to the invention. Such a
characteristic does not exist in the case of a control tread
reinforced with carbon black and ZDMA.
[0169] From all the above results, it may be thought that the
stiffening three-dimensional lattice formed by the unsaturated
metal carboxylic acid salt has lesser solidity in the case of the
tread filled with silica than in the case of the conventional tread
filled with carbon black.
[0170] Due to this relative fragility, stresses of low amplitude,
typical of those experienced during running by the surface part of
the tread, would be sufficient to break the surface resin lattice,
and thus to make the surface part of the tread more flexible and
less rigid, and thus make it recover the excellent grip performance
which it has in the absence of the metal carboxylic acid salt. On
the other hand, in depth, the lattice resin would be little
affected by rolling, all the less so as one penetrates inside this
tread, thus guaranteeing the additional rigidity sufficient for
improved road behavior (greater drift thrust).
[0171] Thus, once accommodated (run in), the tread according to the
invention, which is flexible at the surface and rigid in its depth,
combines the two contradictory demands of road behavior and grip on
wet, snow-covered or icy ground.
[0172] The rigidity gradient described above is illustrated in
particular by a very significant difference in modulus at low
deformation, or in Shore hardness, between the radially outermost
part of the tread, the part in contact with the ground, and the
radially innermost part of this tread (or portion of tread, in the
case for example of a hybrid tread) stiffened by the presence of
the resin lattice.
[0173] After standard running-in of passenger-car tires according
to the invention, the secant modulus ME10 (10% elongation) may thus
vary advantageously from one to two times as much, between the
surface (typically between 2 and 8 MPa, depending on the type of
tire) and the radially innermost part (typically between 4 and 16
MPa, depending on the type of tire) of the tread.
[0174] Comparative Shore hardness measurements carried out on the
surface of treads whether or not in accordance with the invention
typically yield the following results:
1 Carbon black filler: without ZDMA, new tire: between 60 and 70
points; with ZDMA (12 phr), new tire: between 70 and 80 points;
with ZDMA (12 phr), after between 70 and 80 points. standard
running-in: Silica filler: without ZDMA, new tire: between 60 and
70 points; with ZDMA (12 phr), new tire: between 70 and 80 points;
with ZDMA (12 phr), after between 60 and 70 points. standard
running-in:
[0175] The conventional tire the tread of which is reinforced with
carbon black and ZDMA retains its surface rigidity after
running-in, whereas the tire the tread of which is reinforced with
silica and ZDMA recovers its range of initial hardness after
running-in, which confirms the changes in moduli ME 10 commented on
previously.
[0176] The invention thus, unexpectedly, makes it possible to
modulate the differences in rigidity between the surface of a tread
and its radially inner part, and therefore to adjust the desired
compromise of grip/road behavior.
[0177] It is henceforth possible to produce tires of low rolling
resistance the tread of which (at the very least for the portion of
tread comprising a salt such as ZDMA in the case of a hybrid tread
of the "cap-base" type) has a modulus which increases radially in a
continuous profile, which is low at the surface (for example
between 2 and 8 MPa in the case of a passenger-car tire), and high
in the depth (for example between 8 and 16 MPa in its radially
innermost part, for this same passenger-car tire).
[0178] Preferably, in the case of a conventional passenger-car tire
(summer), the modulus ME 10 will thus be of between 5 and 8 MPa (in
particular between 5.5 and 7.5 MPa) at the surface and between 8
and 14 MPa (in particular between 9 and 13 MPa) in the depth
(radially innermost part). Preferably, in the case of a "winter"
tire intended for snow-covered or icy roads, the modulus ME10 will
be of between 3 and 6 MPa at the surface (in particular between 3.5
and 5.5 MPa), and between 6 and 12 MPa (in particular between 7 and
11 MPa) in the depth.
III. EXAMPLE OF EMBODIMENT
[0179] According to a specific example of manufacture of a tread
according to the invention, the procedure is as follows: the
reinforcing filler (for example 80 phr or more of HD silica), the
coupling agent (for example 6 phr or more of TESPT), the diene
elastomer or the mixture of diene elastomers (for example a 70/30
blend of SBR/BR), the unsaturated metal carboxylic acid salt (for
example 5, 10 or 20 phr of ZDMA) and the various other ingredients
(extender oil for the SBR, carbon black in a small amount of about
6 phr, DPG, ZnO, stearic acid, antioxidant in the conventional
amounts), with the exception of the vulcanization system (sulphur
and sulphenamide) and the radical initiator (peroxide), are
introduced in succession into an internal mixer filled to 70% of
capacity, the initial tank temperature of which is approximately
60.degree. C.
[0180] Thermomechanical working (non-productive phase) is then
performed in one stage, of a duration of 3 to 4 minutes in total,
until a maximum "dropping" temperature of 165.degree. C. is
obtained. The mixture thus obtained is recovered, it is cooled and
then the sulphur (for example 1.5 phr), sulphenamide accelerator
(for example 1.5 phr) and peroxide (for example 0.1, 0.5 or 1 phr)
are incorporated on an external mixer (homo-finisher) at 30.degree.
C., by mixing everything (productive phase) for an appropriate
time, of between 5 and 12 minutes.
[0181] The composition thus obtained is then extruded directly in
the form of a radial-carcass passenger-car tire tread, of dimension
195/65 R15 (speed index H).
[0182] The rubber compositions previously described, based on diene
elastomer, reinforcing inorganic filler, a coupling agent and a
unsaturated metal carboxylic acid salt such as ZDMA, in the
proportions indicated above, may advantageously constitute the
entire tread according to the invention.
[0183] However, the invention also applies to those cases in which
these rubber compositions comprising the metal carboxylic acid salt
form only part of a composite tread such as described for example
in the introduction to the present specification, formed of at
least two radially superposed layers of different rigidity
(so-called "cap-base" structure), both intended to come into
contact with the road during rolling of the tire, during the life
of the latter.
[0184] The part comprising the metal carboxylic acid salt may then
constitute the radially outer layer of the tread intended to come
into contact with the ground from the start of rolling of the new
tire, or on the contrary its radially inner layer intended to come
into contact with the ground later on, in the event that it is for
example desired to "delay" the technical effect of
auto-accommodation provided by the invention. Due to the treads
according to the invention and to the specific formulation of their
rubber compositions, it is henceforth possible to "reconcile" grip
on wet ground and road behavior, without using solutions which are
complex, costly or non-durable such as described in the
introduction to the present specification.
[0185] The treads according to the invention offer the major
advantage, compared with the composite treads of the prior art, on
one hand of maintaining their compromise of performances throughout
the life of the tire, due to the unexpected phenomenon of
auto-accommodation which is observed, and on the other hand of
having a true radial rigidity gradient, and not a simple, very
localized, "accident" of rigidity. This true rigidity gradient
results in optimum "working" of the blocks of rubber in contact
with the ground, during rolling and the numerous forces transmitted
to the tread, in other words is synonymous with a tire which grips
the road even better.
[0186] This result may be obtained while maintaining the
performances of rolling resistance and wear resistance at the high
levels which one is entitled to expect nowadays from rubber
compositions based on reinforcing inorganic fillers such as highly
dispersible silicas.
[0187] The invention finds an advantageous application in tires
fitted on vehicles such as motorcycles, passenger cars, vans or
heavy vehicles, in particular in high-grip tires of the "snow" or
"ice" type (also referred to as "winter" tires) which, owing to a
deliberately more flexible tread, could have lower-performance road
behavior on dry ground.
* * * * *